1. Field of the Invention
The present invention relates to a suspension for supporting a mass body on a support base via a parallel combination of a spring means and a shock absorber, and more particularly to a control of such a suspension with certain variations of the damping coefficient of the shock absorber.
2. Description of the Prior Art
A suspension for supporting a mass body on a support base via a parallel combination of a spring means and a shock absorber is common in the art of automobiles, wherein a mass body comprised of a vehicle body and a passenger or passengers is supported on a plurality of vehicle wheels via a plurality of suspension mechanisms each including a parallel combination of a suspension spring and a shock absorber, so that each parallel combination of the spring and the shock absorber bears a share of the total mass of the vehicle body and the passenger or passengers to support it on a corresponding vehicle wheel.
FIG. 13 illustrates schematically such a dynamic system, wherein a mass body 100 having a mass M is supported on a support base 110 via a suspension comprising a parallel combination of a spring 130 having a spring coefficient K and a shock absorber 120 having a damping coefficient C. As is well known in the art, expressing the absolute displacements of the support base 110 and the mass body 100 in reference to a certain absolute ordinate by X and Z, respectively, thereby also expressing the relative displacement of the mass body 100 in reference to the support base 110 by Y, the dynamic motion of the dynamic system is expressed as follows: ##EQU1##
In the suspension shown in FIG. 13, the spring 130 is indispensable for supporting the mass body 100 against the gravity in the dynamic state as well as in the static state of the system, whereas the shock absorber 120 operates only to dampen a dynamic movement of the mass body 110 and is ineffective in the static state of the system. In view of this, it is considered to mount the shock absorber 120 between the mass body 100 and a stationary member 140 as shown in FIG. 14, in order to more effectively dampen the dynamic movement of the mass body 100 when desired, although no such stationary member is available in the case of the suspension of automobile. A damper such as the shock absorber 120 in FIG. 14 provided above the mass body 100 to dampen the movement of the mass body 100 from the overhead stationary member 140 is called a sky hook damper. It has been proposed in Japanese Patent Laid-open Publications 3-276806, 3-276807, 3-276808, 3-276811 and 4-15113 assigned to the same assignee as the present application to analyze the system shown in FIG. 14 in the equivalency conversion to the system shown in FIG. 13, so that the damping action of the shock absorber 120 in the system shown in FIG. 13 is approximated to the shock absorber 120 in the system shown in FIG. 14, so as thereby to obtain a more stable damping effect for the mass body 100 without the stationary member in the case of the automobile suspension.
In more detail, according to the conventional dynamics, the dynamic motion of the system shown in FIG. 14 is expressed by ##EQU2## wherein C* and K are the damping coefficient and the spring coefficient of the shock absorber 120 and the spring 130 in FIG. 14, respectively. Therefore, from the comparison of equations (1) and (2), if the damping coefficient C of the system shown in FIG. 13 is controlled to be a product of a constant damping coefficient C* in FIG. 14 and a ratio Z/Y, i.e. the ratio of the absolute velocity of the mass body 100 to the relative velocity of the mass body 100 in reference to the support base 110, the mass body 100 in the system shown in FIG. 13 will be more stably suspended on the support base 110 as if the dynamic movement of the mass body 100 were dampened by the shock absorber 120 supported by the stationary member 140 in FIG. 14.
According to the above-mentioned sky hook damper theory, the damping coefficient of the shock absorber is controlled to make the product C*.Z/Y to be equivalent to the damping coefficient C of the conventional shock absorber.
However, when the damping coefficient of the shock absorber is variably controlled, the shock absorber will be able to operate more effectively not only so as to dampen the oscillation of a mass body such as a vehicle body according to the above-mentioned sky hook damper theory but also so as more properly to meet with the irregularity conditions of the road surface which require a soft damper, i.e. a relatively low damping coefficient at times but require a hard damper, i.e. a relatively high damping coefficient at other times.